1. Field of the Invention
This invention relates to a laser printer, and more particularly, to means in such a printer for minimizing laser pointing errors.
2. State of the Prior Art
Laser printers are used in the graphic arts for printing on a receiving medium such as film. When such printers are used as color printers, they generally include a separate channel for each of the primary colors. Each channel has a laser which projects a beam of intense and coherent light at a predetermined wavelength. These beams of light are combined by a beam combiner, and the combined light beam forms a spot that is scanned by a rotating polygon across an image plane. The intensity of the light beam in each channel is modulated in accordance with an electrical signal representing image information. For one type of laser, an acoustooptic modulator can be used to modulate the beam.
There must be precise registration of the three light beams at the image plane in order for a color printer to produce high quality prints. In lasers used in such printers, it has been determined that the laser beam tends to originate from a point in a fixed plane located along the optical axis. This plane is not necessarily aligned with any physical part of the laser; however, the plane is frequently positioned at the exit of the laser. Optical alignment of the laser during manufacture is based on the location of the fixed plane and of the point in the plane where the optical beam exits the plane at some angle to the normal to this plane. The beam usually deviates from this initial alignment after usage due to vibrations, thermal changes and other factors. Thus, pointing errors occur which cause the misalignment of the combined three beams at the image plane. Pointing errors can also cause beam intensity modulation as the beam passes through position sensitive components such as acoustooptical modulators; such modulation may induce artifacts in the image.
U.S. Pat. No. 3,974,507 is directed to a micrographics device for machining holes in a thin film by means of a solid-state junction laser. A lens array is interposed between the output face of the laser and the thin film to correct for astigmatism and an elliptical cross section of the output beam of the laser. However, the lens array disclosed in this patent is not suitable to correct for pointing errors in a laser printer of the type disclosed herein.